1. Field of the Invention
The present invention relates to a method of manufacturing a ceramic honeycomb structure which is preferably usable in a filter such as a diesel particulate filter and in which predetermined cells are plugged at end surfaces.
2. Description of the Related Art
As a dust collecting filter represented by a diesel particulate filter (hereinafter referred to as “DPF”), a ceramic filter having a honeycomb structure is used. This filter has a structure of a porous honeycomb structure having a large number of cells partitioned by partition walls and extending in an axial direction. In addition, each cell is plugged at one end surface so that adjacent cells are plugged at opposite end surfaces in such a manner that an end surface has a checkered pattern.
When an exhaust gas is passed through the honeycomb structure from one end surface thereof, exhaust gas including micro particles such as particulates flows into the structure from the cell which is not plugged at the end surface. Then the gas passes through the porous partition wall to enter other cells that are not plugged at the other end surface of the structure. When the gas passes through the partition wall, the partition wall captures the particulates in the exhaust gas. The purified exhaust gas from which the particulates have been removed is discharged from the other end surface of the honeycomb structure.
Usually, when manufacturing the honeycomb structure in which predetermined cells are plugged at the end surface in this manner, as shown in FIG. 9, a method is applied in which a film 4 is attached as a mask to the end surface of a honeycomb structure 1, and holes 3 through the film 4 are made at positions corresponding to openings of predetermined cells 2a, 2b. The end portion of the honeycomb structure 1 is immersed into a container 5 in which plugging slurry 6 is stored. Then the end surface is pressed so that the plugging slurry 6 is pressed into the openings of the predetermined cells 2a, 2b through the holes 3 of the film 4 (see, e.g., JP-2001-300922A).
In a case where the predetermined cells are plugged in the above-described method, diameters of the holes 3 disposed in the film 4 have heretofore been set to be equal. However, when the holes 3 having the equal diameters are simply disposed in the film 4 constituting the mask, a length of the plugging slurry 6 pressed into the respective cells 2a, 2b to be plugged is not uniformed in many cases. In general, as shown in FIG. 9, the plugging slurry pressed into the cells 2a in the vicinity of an outer periphery of the honeycomb structure 1 tends to become shorter in length than that pressed into the cells 2b in a central portion.
Moreover, when the length of the plugging slurry pressed into each cell to be plugged is non-uniform in this manner, pressure loss in each cell fluctuates, and there occurs a disadvantage that an amount of deposition in each cell varies.